1. Field of the Invention
The invention relates to a device used to determine the optical density and color of sunglass or therapeutic tints required for the comfort and optimal visual performance of the person being tested. The device also has been designed for checking the overall condition of the retina, cornea, macula and crystalline lens as regards macular degeneration and color and contrast sensitivity. The device may also find application in detecting retinal integrity as well as cataracts in the crystalline lens and other ocular media. Sunglass lenses and other filtering lenses have been used for centuries, however the determination regarding what sunglass optical filter density and color would be appropriate for a given patient has been left almost entirely to the subjective judgment of the patient and dispenser. Ophthalmic determination of a proper sunglass optical filter density for a patient has only been attempted with bright incandescent lights as the source of high intensity light. This device uses white light emitting diodes to produce a bright light in the vicinity of a specially constructed pattern, enabling the practitioner to determine the required density and color by discovering the lighting conditions required to obscure the particular pattern seen by the patient. It has been determined in studies (Paul de Land, 2000) that contrast sensitivity is dependent on pattern spatial frequency and is altered by the presence of intervening filters of various optical densities and colors. This test, by using patterns with various different spatial frequencies, can potentially be used to determine the required optical filter density and color for the patient to safely detect important patterns such as traffic warning signs. Additionally, such testing may also reveal early stages of macular degeneracy and cataracts of the crystalline lens and other ocular media.
2. Description of Prior Art
Devices similar to this unit are the Brain Power Incorporated Sun Glass Doctor and Solar Sensitivity Meter. These units employ bright incandescent bulbs to supply the bright light used to determine the sunglass density and color level required for specific patients. Sunglasses have been used for centuries and their optical densities in the visible portion of the spectrum have been routinely checked. Determination of the proper density appropriate to the patient using this contrast sensitivity type of test has been attempted by the BPI units previously mentioned. Patients have been evaluated by glare testing in order to obtain related information. Task describes a device wherein light glares through a transparent panel upon which a pattern is placed. McAllister, et al. describe a similar device wherein the glaring light passes through a transparent opening with an opaque pattern surrounding it. Grolman describes a ring of light emitting diodes surrounding a central axis through which the patient attempts to perceive a pattern. The pattern is illuminated and caused to be obscured by the glare of the LED's; the LED's are pointed away from the patient, toward the pattern. Teskey describes a sunlight sensitivity tester wherein a computer controlled illuminated screen provides a test pattern. The screen is then illuminated with a variety of illuminating sources directed at the viewing screen. Filters may be placed between the patient and the viewing screen. Nadler describes an opaque target surrounded by a translucent ring which allows glaring light from an incandescent lamp to pass through, directed at the patient. Ortega, et al., describe a pattern of lit dots surrounding a luminous central glare region.
Objects and Advantages
The device being discussed differs from all such similar devices by the use of light emitting diodes as the source of high intensity light directed at the patient. Focused light emitting diodes are especially useful in this embodiment of the device since the power requirements are further reduced by directing light only at the patient. This power reduction along with the intrinsic power reduction from using light emitting diodes in place of an incandescent bulb leads to a lower operating temperature for the unit and a longer life time for all associated parts. Furthermore, the power line noise produced by modulating the light intensity of a high power incandescent bulb is eliminated by using the light emitting diodes. The light intensity of the diodes is varied by either adjusting the current flowing to them or by time domain modulating that small current using pulse width modulation. The latter technique is the preferred embodiment since the micro-controller which controls the operation of the device easily provides the needed modulating signal. The modulating frequency and current produce a light source that is totally controllable by a computer program. This program helps to produce the variable light stimulus that is needed for spatial frequency determination to check visual function integrity. Each of the light emitting diodes can be individually controlled to produce a sweeping motion across the retina. The use of discrete light emitting diodes allows the light pattern to be shaped to best suit the application. In the preferred embodiment of this device the light emitting diodes are configured in two concentric rings with an overlay containing a target with two opaque, broken, concentric rings nested between the two diode rings as shown in FIG. 1. The target lies against the face plate of the device, which is angled in such a way as to hold the plane of the target perpendicular to the line-of-sight from the patient to the target. Typically, white light emitting diodes with light outputs of several candela are employed in this device and their light is focused toward the patient. Furthermore, this device is not shrouded or covered in such a way as to restrict the patient's peripheral visual field, thus allowing the patient's convergent and accommodative systems to function normally during the examination.